In recent years, a glass material is being used for various uses. Particularly, high-precision surface polishing for a flat and defect-free surface with small surface roughness is demanded for optical lens; a glass substrate of a recording medium for optical disks or magnetic disks; a glass substrate for display panels such as plasma display panel (PDP), thin-film transistor (TFT) type liquid crystal display (LCD) panel and twisted nematic (TN) type LCD panel; a color filter for liquid crystal TVS; and a glass substrate for LSI photomask. As for these glass substrates, a requirement for a reduction in production cost of the substrate along with high-precision surface finishing is increasing.
Incidentally, the display above is used as a monitor for desktop personal computers, or a monitor for liquid crystal televisions, cellular phones, notebook computers, personal digital assistants, digital cameras and the like; and the magnetic disk above is used as a hard disk for mobile devices.
In order to surface polish various types of glass, materials such as cerium oxide, zirconium oxide, iron oxide and silicon dioxide have been conventionally used. In recent years, a cerium oxide-based abrasive mainly comprising cerium oxide has been predominantly used because of high polishing efficiency.
A cerium oxide-based abrasive can be produced by using an ore such as bastnaesite concentrate as the starting material. For example, in the case of producing a cerium oxide-based abrasive by using bastnaesite concentrate as the staring material, impurities in the bastnaesite concentrate are removed by using physicochemical separation, and the residue is then ground to adjust the particle size and further dried. After firing at 600 to 1,000° C. in a rotary kiln or a shuttle kiln, the particle size is again adjusted through disintegration and classification, whereby a cerium oxide-based abrasive is obtained.
Also, recently, there has been proposed a method of grinding a light rare-earth carbonate as the raw material, forming a slurry, adding thereto hydrofluoric acid to effect partial fluorination, and then performing firing (Japanese Unexamined Patent Publication No. 9-183966); a method of calcining a light rare-earth salt to obtain a rare-earth oxide, and subjecting this rare-earth oxide to grinding, treatment with mineral acid and then with ammonium fluoride, and firing (Japanese Unexamined Patent Publication No. 11-269455); and a method of adding a rare-earth fluoride to a rare-earth oxide, and performing wet grinding, drying, firing and classification (Japanese Unexamined Patent Publication No. 2002-224949). Furthermore, in order to not involve the grinding step or greatly simplify the grinding step, a method of adding a precipitant to a cerium-containing light rare-earth salt solution to precipitate fine particles of a light rare-earth salt, thereby obtaining a slurry, adjusting the size of particles in the slurry, subjecting the slurry to fluorination and then to solid-liquid separation, and firing the obtained solid component to produce an abrasive, has been proposed (Japanese Unexamined Patent Publication No. 2002-371267).
However, in the case of fluorinating a light rare-earth salt or a rare-earth oxide obtained by firing a light rare-earth salt as in Japanese Unexamined Patent Publication Nos. 9-183966, 11-269455 and 2002-224949, such a material generally has a large particle diameter and needs to be ground by a wet ball mill or the like. Also, after grinding, a large number of steps must be passed through for drying, firing, disintegration, classification and the like. In general, these steps are performed by a batch system and troublesome task, and the production cost of the abrasive cannot be reduced.
Furthermore, in the case of partially fluorinating a light rare-earth salt or rare-earth oxide particle by adding a fluorine-containing substance such as ammonium fluoride or hydrofluoric acid as in Japanese Unexamined Patent Publication Nos. 9-183966, 11-269455 and 2002-371267, the fluorination reaction proceeds at an extraordinarily high rate in the subsequent firing step to allow for growth of abnormal particles and formation of coarse particles, and the coarse particle may be mixed in the abrasive.